Many methods and devices are known in the art for incorporating photochromic characteristics into an ophthalmic lens. One example of a method known in the art includes imbibing or infusing photochromes into the host material or base lens from a transfer layer (that is subsequently removed) prior to formation of the finished lens product. Another example of a method known in the art includes incorporating photochromes into a lens by imbibing or coating a photochromic composition onto the surface of a base lens. Yet another example of a method known in the art includes incorporating photochromes into a finished lens product by combining a prepared photochromic insert or “laminate” with base lens material, typically via an injection molding process. The following are illustrative examples of such methods known in the art.
The imbibing process was one of the first processes to be used to impart photochromicity to plastic lenses. U.S. Pat. No. 4,286,957 to Naour-Sene describes this process. Further refinements were discussed in U.S. Pat. No. 4,880,667 to Welch. Various improvements to this imbibing process have been developed, such as described in U.S. Pat. No. 5,130,353 to Fisher et. al., and U.S. Pat. No. 5,185,390 to Fisher et. al. These two patents suggest improvements to the transfer process with a unique transfer layer. It was recognized early on that the plastic resins used to make ophthalmic lenses do not provide the best host material for photochromes. In such plastic resin materials, the photochromes do not activate easily and fatigue or wear-out in a short period of time. A strong activation darkness to near sunglass darkness is desired in the marketplace. Another desired characteristic of a photochromic lens is that it should maintain at least 70 percent of its original activation darkness after two years of wear. This is one of the limitations to putting photochromes into polymeric host materials that are used to form the bulk of the lens.
A more recent example, U.S. Pat. No. 5,728,758 to Smith describes a photochromic article in which the organic polymeric host material has been impregnated with photochromes prior to formation of the finished lens product. As is described in the '758 patent, one of the drawbacks of incorporating photochromes directly into the polymeric host material is the problem of fatigue or light fatigue. Photochromes are believed to lose their ability to change color specifically resulting from the irreversible decomposition of the photochromic compound, which occurs due to repeated exposure to UV light over time. The '758 patent specifically address this problem by using a unique combination of monomers and surface coating compositions to improve abrasion resistance, chemical attack and improved fatigue resistance.
Alternatively, an example that describes coating of a photochromic layer onto the surface of a lens is found in U.S. Pat. No. 4,756,973 to Sakagami et al. The '973 patent specifically describes the use of spirooxazine compound and phenolic compound in the photochromic layer, and describes that such a lens formulation provides successful coloring effects in photochromic lenses that are subjected to environmental conditions ranging from normal to high temperatures.
Another example that describes coating photochromes on the surface of a lens substrate is found in U.S. Pat. No. 6,150,430 to Walters et al. Specifically, the '430 patent describes a process that includes the steps of treating the surface of a polymeric substrate to provide reactive groups, applying a polymerizable composition to the surface, exposing the coated substrate to radiation to improve adhesion, and applying and curing a photochromic composition to the coated surface. The '430 patent, at least in part, addresses a method of producing commercially acceptable “cosmetic” standards for photochromic and non-photochromic optical coatings that are applied to lenses. A major limitation of the photochromic coating approach is the poor scratch resistance of such a coating even with another hard coating on top of the photochromic coating. Additionally, if the photochromic coating is scratched, it will result in streaks of areas on the lens that do not activate.
The limitations of the performance of the photochromes in the various plastics used to make ophthalmic lenses have resulted in various improvement methods, including making composite or multiple part lenses that combine plastics that are good photochromic hosts with additional plastics to make improved ophthalmic lenses. One example of this approach is described in U.S. Pat. No. 5,531,940 to Gupta et. al. Another approach is to put the photochromic dyes in the glue layer between two lens sections as described in U.S. Pat. No. 5,851,328 to Kohan. More recent attempts at making photochromic composites are described in U.S. Pat. Nos. 6,863,844 and 6,863,848 to Engardio et. al. and U.S. Publication No. 20050089630 to Schlunt et. al. One problem with these approaches is that the mechanical stability of the composite is not very durable in subsequent processing to make the ophthalmic lens and mount it into a frame. Processes such as surfacing the lens to prescription power and edge cutting to fit into a frame result in chipping of the composite due to the different cutting and grinding characteristics of the materials. Drilling of the composite to mount into rimless frames also results in chipping of the composite.
Lastly, the following methods known in the art illustrate incorporation of photochromes into ophthalmic lenses via photochromic inserts or laminates, whether by cast-mold-type processes or by injection molding processes. For example, U.S. Pat. No. 4,889,413 to Ormsby et al. describes creation of a finished laminate product that is created by placing two glass or plastic layers into a mold and injecting a photochromically-infused plastic resin between the glass/plastic layers. The resulting photochromic laminate is thereafter cured and processed, producing a finished lens product.
Another example that illustrates the use of a photochromic insert or laminate in an injection molding process is described in U.S. Pat. No. 6,328,446 to Bhalakia et al. The photochromic laminate or wafer includes inner and outer resin sheets (or protective layers), which sandwich a photochromic cellulose acetate butyrate layer. The unitary photochromic laminate is thereafter placed inside a mold cavity, after which a molten polycarbonate resin is injected into the cavity and fused to the back of the photochromic laminate. The lens is then cooled to room temperature and the finished product is an injection molded, photochromic polycarbonate lens.
While each of the above-referenced patents and published applications describe methods of making photochromic lenses and address particular problems in the art, improvements are still required. For example, problems associated with impregnating photochromes within the host material of a base lens have been described to some degree above. Additionally, if such a lens is a semi-finished product and requires further processing (e.g., grinding, polishing, etc), it is clear that photochromes present in the base lens will be ground and/or polished away, inevitably diminishing the desired coloring effects of the finished lens product. In addition, the prescription lens must be robust enough to maintain its integrity through subsequent processing to both form the prescription and be edged, cut and possibly drilled for mounting into a frame.
Alternatively, the shortcomings of coating photochromic products onto the surface of a lens have to do primarily with coating thickness and the creation of segmented, multi-focal lenses. For example, a coating of about 25 μm or more is needed to incorporate a sufficient amount of photochromic compounds to provide the desired light blocking quality in the lens when the compounds are activated. However, a coating of this thickness is not well suited for application on the surface of a segmented, multi-focal lens because it is too thick. Typically, a coating of this thickness creates such problems as the creation of an unacceptable segment line, as well as coating thickness uniformity issues around the segment line.
Problems that have been raised particularly regarding use of photochromic laminates or inserts in injection molding process include, primarily, the bleeding of the functional layer (e.g., photochromic layer) material of the laminate or wafer. By the term “bleeding,” it is meant that the functional layer materials between the transparent resin sheets (e.g., the protective layer of the laminate or wafer) runs out from between the resin sheets in the lateral direction.
Often bleeding occurs due to the deformation of the photochromic layer under the high temperature and pressure used during the injection molding process. This is thought to occur due to either an excess amount of functional layer material and/or inadequate softening properties of the functional layer material. Further, this bleeding can interfere with any additional coating layers that are applied to the lens after injection-molding. The Bhalakia patent adequately addresses the issue of making laminates used in injection-molding, through improvement of laminate materials and properties. However, the issues addressed by Bhalakia do not include providing a laminate or insert that may be used in a cast- lens manufacturing process.
Therefore, a need exists to create a photochromic lens that addresses the problem of maximizing photochromic properties of a lens produced in a cast-mold manufacturing process.